Matplotlib：如何垂直对齐多个图例

Question

I have a plot which has three legends of different length and width. I'm trying to find a way to place them nicely onto the plot. Currently I'm placing them with the default loc= which is great for dealing with the vertical placement. The issue is that by default the legends are right aligned, which looks messy.

Is there a way to use the default loc= to place them on the plot, but to have them left aligned?

Example: From the legend guide .

import matplotlib.pyplot as plt

line1, = plt.plot([1,2,3], label="Line 1", linestyle='--')
line2, = plt.plot([3,2,1], label="Line 2\nThis is a \nvery long\nlegend", linewidth=4)
line3, = plt.plot([2,2,2], label="Can this be left justified?")

# Create a legend for the first two lines.
# 'loc' puts them in a nice place on the right.
first_legend = plt.legend(handles=[line1], loc=1)
second_legend = plt.legend(handles=[line2], loc=5)

# Add the legends manually to the current Axes.
ax = plt.gca().add_artist(first_legend)
ax = plt.gca().add_artist(second_legend)

# Create another legend for the last line.
plt.legend(handles=[line3], loc=4)

plt.show()

Which gives this

Now what I would really like is for the legends to left aligned but still on the right side of the plot. Like so:

I know I can place them at a specific location but to do this I need to specify both the x and y coords, which will be fiddly since all 3 have variable heights and widths.

Answer 1

You could use bbox_to_anchor to position legends precisely where you want them:

fig, ax = plt.subplots()
line1, = ax.plot([1,2,3], label="Line 1", linestyle='--')
line2, = ax.plot([3,2,1], label="Line 2\nThis is a \nvery long\nlegend", linewidth=4)
line3, = ax.plot([2,2,2], label="Can this be left justified?")

# Create a legend for the first two lines.
# 'loc' sets anchor position of the legend frame relative to itself, 
# bbox_to_anchor puts legend's anchor to (x, y) in axes coordinates.
first_legend = ax.legend(handles=[line1], loc='upper left', bbox_to_anchor=(0.65, 1))
second_legend = ax.legend(handles=[line2], loc='center left', bbox_to_anchor=(0.65, 0.5))

# Add the legends manually to the current Axes.
ax.add_artist(first_legend)
ax.add_artist(second_legend)

# Create another legend for the last line.
ax.legend(handles=[line3], loc='lower left', bbox_to_anchor=(0.65, 0))

plt.show()

The only number you would need is x position of the legends' bbox_to_anchor to align to ( 0.65 in the example above).